1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a system for forming an alignment layer of an LCD device and an operation method thereof.
2. Background of the Related Art
In general, cathode ray tubes (CRTs) have been utilized in display devices for displaying image information, but they usually are large and are heavy. With the development of electronic industries, display devices are not only utilized for televisions (TV) and monitors, but also are utilized in personal computers, wireless terminals, vehicle instrument panels, electronic display boards, and the like. Now, with the development of information communication technology, the market has a great demand for the display devices that are capable of processing and displaying large capacity image information. Such display devices are also required to have light weight, thin profile, a high luminance, a large-sized screen, low power consumption, and a low price. For this reason, liquid crystal display (LCD) devices have attracted considerable attention. The LCD device exhibits an excellent resolution among the flat display devices as well as a rapid response time in implementing a moving picture as compared to that of the CRT.
One LCD that enjoys a great popularity is a twisted nematic (TN) mode LCD. In the TN mode LCD, electrodes are formed on two substrates, respectively, and liquid crystal directors are aligned and twisted by 90°, so that a driving voltage is applied to the electrodes to drive the liquid crystal directors. However, the TN mode LCD has the disadvantage of a narrow viewing angle. Recently, many efforts have been made to improve the TN mode LCD. For example, an in-plane switching (IPS) mode LCD and an optically compensated birefringence (OCB) mode LCD have been proposed. The IPS mode LCD generates a horizontal electric field to drive liquid crystal molecules in a horizontal state with respect to the substrates by forming two electrodes on the same substrate and applying a voltage between the two electrodes. In other words, the longer axis of the liquid crystal molecule does not stand up with respect to the substrate. For this reason, the IPS mode LCD has a small variation in the birefringence of liquid crystal according to a visual direction and thus has an excellent viewing angle characteristic compared with the TN mode LCD.
FIG. 1 is a flowchart schematically illustrating a method, including Steps S100 to S180, for fabricating a related art LCD. As illustrated in FIG. 1, upper and lower substrates are prepared at Step S100. The upper substrate includes a color filter, and the lower substrate includes matrix-type pixels with thin film transistors (TFTs). At Step S110, the substrates are cleaned to remove foreign substances thereon. At S120, a polyimide material is printed on a top surface of each substrate to form an alignment layer using an apparatus for printing the alignment layer. At Step S130, high-temperature heat is applied to the polyimide material, thereby drying and hardening solvent of the alignment layer. At Step S140, a surface of the hardened alignment layer is rubbed in one direction using a rubbing apparatus. At Step S150, an adhesive seal pattern is formed at an edge of the upper substrate at a region except a liquid crystal injection hole, and spacers are then dispersed on the lower substrate. At Step S160, the lower and upper substrates are bonded together with an accuracy of several micrometers to prevent light leakage. At Step S170, the bonded substrates are cut into unit cells by a cutting process, which is performed for cutting the bonded two substrates into a plurality of unit cells with desired size. This cutting process includes a scribing process for forming lines on the upper and lower substrates and a breaking process for dividing the scribed substrates into unit cells by applying an impact thereon. At Step S180, liquid crystal is injected through the injection hole into a gap between the lower and upper substrates, and then the injection hole is sealed to complete a desired LCD.
A liquid crystal dispensing method may be used instead of the liquid crystal injecting method described above. In this case, the physical properties of the liquid crystal are changed by a molecular arrangement state thereof, thereby making a difference in a response to an external force such as an electric field. Because of the characteristics of the liquid crystal molecule, a control technique for an arrangement state of the liquid crystal molecule is essential for the study on the physical properties of the liquid crystal and the construction of the LCD as well. Specifically, a rubbing process for uniformly aligning the liquid crystal molecules in one direction is essential for a normal driving of the LCD and a uniform display characteristic thereof. A related art alignment layer forming process for determining an initial alignment direction of liquid crystal molecules will now be described below in detail.
The forming process of an alignment layer includes a process of depositing a high polymer thin layer and a process of aligning an alignment layer in one direction. The alignment layer may be made mainly of an organic material of polyimide series and may be aligned mainly through a rubbing process. The organic material of polyimide series is deposited on a substrate, and then a solvent thereof is volatilized at about 60˜80° C. After that, the deposited material is hardened at about 80˜200° C. to form an alignment layer. The alignment layer is rubbed in one direction with a roller having a rubbing cloth such as velvet wound therearound to form an alignment direction thereof. The rubbing process enables an easy and stable alignment process and is thus suitable for mass production of the LCD.
However, the rubbing process may bring about a problem when the rubbing cloth becomes defective during the rubbing operation. Specifically, the rubbing process is performed such that the rubbing cloth and the alignment layer are in a direct contact. Thus, such a rubbing process may cause the contamination of a liquid crystal cell due to particles, damage to a TFT due to an electrostatic discharge, and a non-uniform alignment of liquid crystal in a wide-screen LCD, thereby decreasing the production yields of the LCD. Moreover, an additional cleaning process is needed after the rubbing process.
FIGS. 2A and 2B are a sectional view and a plan view schematically illustrating an alignment state of liquid crystal around a stepped portion of an electrode pattern such as a pixel electrode and a common electrode in the related art LCD. Recently, an IPS mode LCD with an improved viewing angle and an IPS mode LCD manufactured using 3 or 4 masks to reduce the number of manufacturing processes have been developed. Unfortunately, in such an IPS mode LCD, a step difference of the stepped edge portion is increased, thereby making the alignment defect worse.
As shown in FIGS. 2A and 2B, an alignment layer 151 is formed on a pixel electrode 130 patterned on a lower substrate. The pixel electrode 130 has a stepped edge portion with a predetermined step difference. A color filter layer 160 and an alignment layer 152 are formed on an upper substrate facing the lower substrate. A liquid crystal layer 190 is formed between the upper and lower substrates. The stepped portion at the corners of the pixel electrode 130 in the pixel region may cause a non-uniform alignment of liquid crystal, thereby bringing about a problem in driving the liquid crystal. Specifically, if the liquid crystal is in a normally-black mode, a black color is displayed when no voltage is applied. Thus, light leakage occurs in a region A as shown in FIGS. 2A and 2B when no gate voltage is applied. That is, when no voltage is applied, the liquid crystal should be aligned in the same direction as the rubbing direction of the alignment layers 151 and 152. However, the stepped edge portion of the pixel electrode 130 causes a distortion phenomenon in which the liquid crystals have an alignment direction different than the rubbing direction. As a result, when no voltage is applied in the normally-black mode, light of the backlight assembly passes through the region A. This causes light leakage in a black display state and a decrease in a contrast ratio, thereby making it difficult to implement a high image quality.